Not Applicable.
This invention relates noise cancellation and more particularly to technique and apparatus for canceling non-acoustic self noise associated with mechanically induced vibrations.
As is known in the art, a towed-array type of sonar system typically includes an array of sensors, such as hydrophones which are physically coupled by a first line or cable (e.g. a fluid-filled hose) which is typically provided from a material having strength suitable to pull the array and typically of neutral buoyancy. A second line or cable (again provided from a suitable material) physically couples the first line, and hence the array of sensors, to a tow vehicle (e.g. a ship) which tows the sensor array through a medium such as water.
As is also known, a sonar array towed through a medium in this manner typically generates non-acoustic noise referred to as non-acoustic self noise. Self-noise is generally due to mechanical vibrations induced by hydrodynamic flow over the array sensors as the array is towed through the medium in which it is disposed. The vibrations propagate as transverse and longitudinal modes in the array body, much like a vibrating string with fixed boundary conditions. The vibrations produce local accelerations at each sensor (e.g. the pressure head of a hydrophone). The acoustic response-induced by this phenomenon can be several orders of magnitude stronger that that of acoustic signals propagating through the water column.
Such non-acoustic self-noise observed on sonar towed arrays can unnecessarily prevent the detection and discrimination of acoustic signatures at low frequency, and thus can impose limits on system performance beyond those implied by the ambient acoustic noise environment. Interference resultant from non-acoustic self-noise tends to dominate the dynamic range of the equipment (e.g. a lofargram display) used by a sonar analyst to classify signatures.
Various strategies have been used to overcome the limitations due to self noise. For example, mechanical self-noise suppression techniques such as vibration isolation and cable fairing are capable of attenuating the propagation of vibrations in the array body. These approaches usually require a mechanical connection or physical alteration to the first cable which couple the array of sensors together and to the to cable which couples the array of sensors to the tow vehicle.
Also, signal processing techniques such as noise spectrum equalization are also employed to provide some dynamic range compression at the display level. However, this approach is an incoherent technique, i.e. ignores phase, and thus comes at some cost to signal of interest (SOI) detectability.
It would therefore, be desirable to provide a system which rejects vibrational self-noise without requiring added sensors or external connections to the sensor array or to cables coupled to the sensor array.
In accordance with the present invention, a device for canceling non-acoustic self-noise associated with mechanically induced vibrations in a sensor array includes an acoustic signal-processing path having an input coupled to the sensor array and adapted to detect signals having a first predetermined range of phase speed, a noise signal processing path coupled to the sensor array and adapted to detect signals having a phase speed (or phase speeds) which is (are) different than the first predetermined phase speed such that the noise signal processing path isolates vibrational noise from the sensor array and a combiner circuit coupled to receive a first signal from the acoustic signal-processing path and a second signal from the noise signal-processing path and for combining the first and second signals to produce a clean output signal.
With this particular arrangement, a device which rejects vibrational self-noise is provided. The non-acoustic noise canceller coherently rejects vibrational self-noise by recognizing that most vibrational modes of a towed sensor array propagate at phase speeds substantially less than that of information-bearing acoustic signals in a water column. This permits the formation of an adaptive interference reference beam steered to non-acoustic wavenumber space, or that part of frequency-wavenumber space within which the signal of interest is known not to exist. It should be appreciated that the present invention will work with any array subject to mechanical vibrations or other high wavenumber noise relative to the signal of interest. Thus, a signal-free, interference reference beam is formed from the same sensor which samples the data, with no need for additional measurement channels such as accelerometers or strain gauges to independently sense the undesirable distortions caused by cable strumming. The reference beam should always be directed into non-acoustic space to avoid injection of acoustic artifacts into water-borne acoustic cone. In one embodiment, the reference beam is directed along an axis of the sensor array (i.e. the reference beam is provided as an end-fire beam), tuned to phase speed of 1000 m/s.
In a further embodiment, the device includes a plurality of acoustic signal-processing paths and one or more noise signal processing paths each of which are coupled to the sensor array. Each of the acoustic signal-processing paths are adapted to detect signals having a first predetermined range of phase speed; while each of the one or more noise signal processing paths are adapted to detect signals having a range of phase speed which is different than the first predetermined phase speed. In this manner, each of the one or more noise signal processing paths isolates vibrational noise from the sensor array.
The recognition that most vibrational modes of an array (e.g. a sonar array) propagate at a phase speed substantially less than that of acoustic signals in the water column which permits the formation of an adaptive interference reference beam steered to non-acoustic wavenumber space leads to the architecture. Such an architecture provides for the coherent cancellation of non-acoustic self-noise.
In accordance with a further aspect of the present invention, a method for canceling non-acoustic self-noise associated with mechanically induced vibrations in a sensor array includes the steps of: (a) providing two or more signals from the sensor array to an input of an acoustic signal processing path, (b) weighting the signal with a first set of weighting coefficients selected to form a first beam to detect signals at a particular arrival angle relative to the sensor array (c)providing two or more signals from the sensor array to an input of a noise signal-processing path; and (d) weighting the two or more signals with a second different set of weighting coefficients selected to isolate signals having a phase speed different than the phase speed of the signals detected by the first beam. The method utilizes the isolation of a vibration reference via digital beamforming to non-acoustic wavenumber space. This attribute forms the basis for the method which produces an interference reference measurement which is sensitive only to vibrations propagating within an array body and independent of acoustic signals propagating in surrounding medium. The approach is distinguished from the prior art in that reference isolation requires no extraneous sensors such as acclerometers, strain gauges, or fiber optic or piezoelectric sensing devices. The approach is also distinguished from the prior art in that it is aimed at canceling self-noise induced by hydrodynamic flow over the body of the sensor array itself, not own-ship radiated propulsion or machinery noise. In one embodiment, the cancellation system employs an adaptive sidelobe canceller, to coherently subtract an appropriately weighted function of the vibration reference measurement from the acoustic channel of interest. The technique of the present invention thus exploits phase and the propagation physics underlying sensor array-borne mechanical vibrations to spatially reject this form of broadband interference. Specifically, it has been recognized that most vibrational modes of an array (e.g. a sonar array) propagate at a phase speed substantially less than that of acoustic signals in the water column. This permits the formation of an adaptive interference reference beam steered to non-acoustic wavenumber space.
It should be appreciated that the apparatus and method described herein, while explained in the context of passive processing of sonar towed array data subject to cable strum self-noise, also find application in a wide variety of areas including but not limited to: (1) cable strum rejection in towed arrays used in marine seismic prospecting or active sonar processing in general; (2) cable strum rejection for vertical line arrays subject to ocean currents used in systems for global monitoring ocean temperature using acoustic transmissions; (3) ground roll rejection in near-surface seismic exploration for characterization of underground structures, mine detection, and archeological investigation; and (4) vibration rejection in multimedia microphone arrays used in speaker recognition and isolation